Trapezoidal back bias and trilayer reader geometry to enhance device performance

ABSTRACT

A magnetoresistive sensor having a trilayer sensor stack with two ferromagnetic freelayers separated by a nonmagnetic spacer layer is disclosed. The sensor is biased with a back biasing magnet adjacent a back of the trilayer sensor. The back biasing magnet, the trilayer sensor stack, or both have substantially trapezoidal shapes to enhance the biasing field and to minimize noise. In some embodiments, the trilayer sensor or back bias magnet have a shape designed to stabilize a micromagnetic “C” shape or concentrate magnetic flux in the trilayer sensor stack.

BACKGROUND

In a magnetic data storage and retrieval system, a magnetic recordinghead typically includes a reader portion having a magnetoresistive (MR)sensor for retrieving magnetically encoded information stored on amagnetic disc. Magnetic flux from the surface of the disc causesrotation of the magnetization vector of a sensing layer or layers of theMR sensor, which in turn causes a change in electrical resistivity ofthe MR sensor. The sensing layers are often called “free” layers, sincethe magnetization vectors of the sensing layers are free to rotate inresponse to external magnetic flux. The change in resistivity of the MRsensor can be detected by passing a current through the MR sensor andmeasuring a voltage across the MR sensor. External circuitry thenconverts the voltage information into an appropriate format andmanipulates that information as necessary to recover the informationencoded on the disc.

MR sensors have been developed that can be characterized in threegeneral categories: (1) anisotropic magnetoresistive (AMR) sensors, (2)giant magnetoresistive (GMR) sensors, including spin valve sensors andmultilayer GMR sensors, and (3) tunneling giant magnetoresistive (TGMR)sensors.

Tunneling GMR (TGMR) sensors have a series of alternating magnetic andnon-magnetic layers similar to GMR sensors, except that the magneticlayers of the sensor are separated by an insulating film thin enough toallow electron tunneling between the magnetic layers. The resistance ofthe TGMR sensor depends on the relative orientations of themagnetization of the magnetic layers, exhibiting a minimum for aconfiguration in which the magnetizations of the magnetic layers areparallel and a maximum for a configuration in which the magnetizationsof the magnetic layers are anti-parallel.

For all types of MR sensors, magnetization rotation occurs in responseto magnetic flux from the disc. As the recording density of magneticdiscs continues to increase, the width of the tracks as well as the bitson the disc must decrease. This necessitates increasingly smaller MRsensors as well as narrower shield-to-shield spacings. As MR sensorsbecome smaller in size, particularly for sensors with dimensions lessthan about 0.1 micrometers (μm), the sensors have the potential toexhibit an undesirable magnetic response to applied fields from themagnetic disc. MR sensors must be designed in such a manner that evensmall sensors are free from magnetic noise and provide a signal withadequate amplitude for accurate recovery of the data written on thedisc.

GMR and TGMR readers can use the resistance between the freelayer and areference layer to detect media stray fields so as to read back storedinformation. Magnetization of the reference layer is fixed through anantiferromagnetic coupling interaction by a ferromagnetic pinned layerwhich is again pinned by antiferromagnetic (AFM) material. The referenceand the pinned layer, together with the antiferromagnetic coupling layerbetween them, are the so-called synthetic antiferromagnetic (SAF)structure. This kind of configuration has two major disadvantages. Thefirst one is high shield-to-shield spacing due to the complicatedmulti-layer structure. The continued reduction of the shield-to-shieldspacing requirement is limited by the emerging instability of individuallayers in the sensor as they become thinner. For example, the pinningstrength of the AFM materials decreases with a reduction in theirthickness. As a consequence, weakly pinned SAF structures lead to anincrease of sensor noise when the reference layer is not satisfactorilypinned. The second disadvantage of traditional GMR and TGMR sensors istheir low sensitivity because the freelayer is the only response layer.Reducing the free layer thickness correspondingly reduces thesensitivity.

Trilayer readers with dual free-layers are one solution to address theseissues. In a trilayer structure, two free-layers with easy axes ofmagnetization in a scissor orientation are used to detect media magneticflux. Synthetic antiferromagnetic (SAF) and antiferromagnetic (AFM)layers are not needed and free layer biasing comes from the combinationof backend permanent magnet and demagnetization fields when bothfreelayers have ends at the air bearing surface. However, the biasingfield from the back end magnet decays rapidly away from the magnet. Thefreelayer portion of the trilayer sensor in the vicinity of the airbearing surface (ABS) suffers from insufficient bias and themagnetization scissor angle is open too much.

SUMMARY

A magnetoresistive sensor includes a trilayer sensor stack comprisingtwo ferromagnetic freelayers separated by a nonmagnetic spacer layerwith a front width proximate an ABS, and a back width distal from an ABSand a back biasing magnet with a trapezoidal shape with a front widthand a back width. The front width of the biasing magnet is adjacent theback width of the trilayer sensor stack and is about the same as theback width of the sensor stack. The back width of the biasing magnet islarger than the front width. The trilayer sensor stack can have arectangular shape or a trapezoidal shape wherein the back width islarger than the front width. The trapezoidal shape concentrates themagnetic field at the front of the biasing magnet in the vicinity of thesensor stack. The trapezoidal shape also encourages “C” typemicromagnetic magnetization patterns in the trilayer sensor stack,minimizing signal noise due to “C” to “S” switching during sensoroperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram showing micromagnetic magnetizationpatterns in a rectangular sample.

FIG. 1B is a schematic diagram showing a “C” type micromagneticmagnetization pattern in the sample of FIG. 1A.

FIG. 1C is a schematic diagram showing an “S” type micromagneticmagnetization pattern in the sample of FIG. 1A.

FIG. 1D is a schematic showing a “C” type micromagnetic magnetizationpattern in a trapezoidal sample.

FIG. 2 is a top view of a first example of a read head in accord withthe present invention.

FIG. 3 is an ABS view of the read head in FIG. 2 in accord with thepresent invention.

FIG. 4A is a schematic top view of the trilayer sensor in FIG. 2 showingbiasing in the absence of external bit flux.

FIG. 4B is a schematic top view of the trilayer sensor in FIG. 4A underthe influence of a first state of data.

FIG. 4C is a schematic top view of the trilayer sensor in FIG. 4A underthe influence of a second state of data.

FIG. 5 is a top view of a second example of a read head in accord withthe present invention.

FIG. 6 is an ABS view of the read head in FIG. 5 in accord with thepresent invention.

FIG. 7A is a schematic top view of the trilayer sensor in FIG. 5 showingbiasing in the absence of external bit flux.

FIG. 7B is a schematic top view of the trilayer sensor in FIG. 7A underthe influence of a first state of data.

FIG. 7C is a schematic top view of the trilayer sensor in FIG. 7A underthe influence of a second state of data.

FIGS. 8A-8K illustrate the fabrication steps to produce the read headillustrated in FIGS. 2 and 3.

FIGS. 9A-9K illustrate the fabrication steps to produce the read headillustrated in FIGS. 5 and 6.

DETAILED DESCRIPTION

The inventive shapes disclosed herein increase the performance of areader by increasing the bias field at the front of a back bias magnetand by decreasing signal noise. The origin of these effects is shown inFIGS. 1A-1C. FIG. 1A illustrates possible micromagnetic magnetizationpatterns in a rectangular magnetic sample under a magnetization orientedgenerally from the left to right. Magnetization vectors 12′ and 14′originate at the corners of the sample and are directed to the centerwhere they converge at magnetization vector 10′. Magnetization vector10′ diverges into vectors 16′ and 18′ as it approaches the right side ofthe sample. FIG. 1 shows all possible micromagnetic magnetizationpatterns. Two patterns are energetically favored. FIG. 1B illustrates a“C” pattern comprised of vectors 12′, 10′ and 16′. An alternative “C”pattern comprises vectors 14′, 10′ and 18′. FIG. 1C illustrates an “S”pattern comprised of vectors 12′, 10′ and 18′ or alternatively vectors14′, 10′ and 16′. The energy difference between the “C” state and the“S” state is very small and during magnetic switching, thermallyactivated transitions between both patterns contribute to measurablesensor noise.

By changing the geometry of a magnetic element, one or the other of the“C” and “S” states can be energetically favored. FIG. 1D illustrates howthe “C” state can be favored by a trapezoidal shape of the micromagneticelement. This shape will be used in what follows to tailor magnetizationin the back bias permanent magnet of a trilayer reader as well as in thefreelayers of the reader itself. Although trapezoidal geometries arediscussed herein to favor “C” shape micromagnetic magnetizationpatterns, it should be noted that other geometries such as half moonshapes can be used to obtain similar beneficial results.

FIGS. 2 and 3 illustrate one aspect of the trilayer reader of thepresent invention. FIG. 2 is a top view of trilayer read head 10, andFIG. 3 is an ABS view of read head 10. Read head 10 comprisesrectangular trilayer reader stack 20 (comprising ferromagneticfreelayers 22 and 24 and spacer layer 26) in front of trapezoidal backbias magnet 30. Magnetic side shields 40 and 42 abut both sides of biasmagnet 30 and trilayer reader stack 20. Trilayer reader stack 20, biasmagnet 30, and side shields 40 and 42 are separated from each other byinsulating layer 50. Side shields 40 and 42 may also be replaced by aninsulator preferably an oxide of aluminum.

The ABS view of trilayer read head 10 in FIG. 3 shows top shield 60,bottom shield 70 and side shields 40 and 42 adjacent trilayer readerstack 20 and insulator layer 50. Ferromagnetic freelayers 22 and 24 oftrilayer reader stack 20 are separated by spacer layer 26. If spacerlayer 26 is a nonmagnetic electrical conductor, read head 10 is a GMRhead. If spacer layer 26 is a nonmagnetic electrical insulator, readhead 10 is a TGMR head. Read head 10 can be a current perpendicular toplane (CPP) head wherein electrical contact is made to trilayer readerstack 20 through top shield 60 and bottom shield 70.

If spacer layer 26 is nonmagnetic, and electrically conducting, it maybe fabricated from, for example, copper. If spacer layer 26 isnonconducting, it may be fabricated from, for example, aluminum oxide(Al₂O₃ or Al_(x)O where x may or may not be an integer) or magnesiumoxide. Ferromagnetic layers 22 and 24 may be fabricated from magneticmaterial such as, for example, nickel-iron-cobalt (Ni—Fe—Co)compositions. The shield layers may be fabricated from, for example, asoft magnetic material such as nickel-iron (Ni—Fe). Back bias magnet 30may be fabricated from a permanent magnet material such as, for example,a cobalt-platinum (Co—Pt) alloy.

The operation of read head 10, according to one aspect of the inventionis described in conjunction with FIGS. 4A-4C. FIGS. 4A, 4B and 4C showtop views of read head 10 with magnetization vector 30′ of back biaslayer 30 oriented with respect to magnetization vectors 22′ and 24′ offreelayers 22 and 24 to achieve optimum response of freelayers 22 and 24to external magnetic fields. In the absence of back bias magnetization,freelayer magnetization vectors 22′ and 24′ would be antiparallel andcommonly parallel to the ABS. Under the bias of magnetization vector30′, they arrange in a scissor orientation for optimum sensitivity. Onebenefit of the trapezoidal shape of back bias magnet 30 is that thesmaller base near the back of trilayer reader stack 20 results inmagnetic flux concentration in that region resulting in deeperpenetration of the biasing field into reader stack 20 in the directionof the ABS.

FIGS. 4A-4C illustrate the effect of varying bit magnetization onrecorded media on the magnetization directions 22′ and 24′ of firstfreelayer 22 and second freelayer 24 respectively. FIG. 4A showstrilayer reader stack 10 in a quiescent magnetic state when it is notunder the influence of magnetic flux emanating from recording media. Theangle of magnetization between first ferromagnetic freelayer 22 andsecond ferromagnetic freelayer 24 at the ABS is in a scissors relationfor optimum sensor response. FIG. 4B is a top view of read head 10showing trilayer reader stack 20 under the influence of a first state ofdata D1 corresponding to a positive bit. This first state of data causesthe angle of magnetization between first freelayer 22 and secondfreelayer 24 to increase at the ABS. When this occurs, the resistanceacross trilayer reader stack 20 changes and is detected when a sensecurrent is passed through trilayer reader stack 20. FIG. 4C is a topview of read head 10 showing trilayer reader stack 20 under theinfluence of a second state of data D2 corresponding to a negative bit.This second state of data causes the angle of magnetization betweenfirst freelayer 22 and second freelayer 24 to decrease at the ABS. Aswith the first state of data, the second state of data causes a changein resistance across trilayer reader stack 20 and is detected when asense current is passed through trilayer reader stack 20.

FIGS. 5 and 6 illustrate another aspect of the invention. FIG. 5 is atop view of trilayer reader head 110, and FIG. 6 is an ABS view of readhead 110. Read head 110 comprises trapezoidal trilayer reader stack 120comprising ferromagnetic freelayers 122 and 124 and spacer layer 126 infront of trapezoidal back bias magnet 130. Magnetic side shields 140 and142 are adjacent both sides of back bias magnet 130 and freelayer stack120. Trilayer reader stack 120, back bias magnet 130, and side shields140 and 142 are separated from each other by insulating layer 150. Sideshields 140 and 142 may also be replaced by an insulator, preferably anoxide of aluminum. In this aspect of the invention, trilayer readerstack 120 has a trapezoidal shape. A benefit of the trapezoidal shape isthat a “C” pattern of micromagnetic magnetization in reader stack 120 ispreferred. The ABS view of trilayer read head 110 in FIG. 6 shows topshield 160, bottom shield 170 and side shields 140 and 142 adjacenttrilayer reader stack 120 and insulator layer 150. Ferromagneticfreelayers 122 and 124 of trilayer reader stack 120 are separated byspacer layer 126. If spacer layer 126 is nonmagnetic, read head 110 is aGMR head. If spacer layer 126 is an insulator, read head 110 is a TGMRhead. Read head 110 can be a current perpendicular to plane (CPP) headwherein electrical contact is made to trilayer reader stack 120 throughtop shield 160 and bottom shield 170.

If spacer layer 126 is nonmagnetic and electrically conducting, it maybe fabricated from, for example, copper. If spacer layer 126 isnonconducting, it may be fabricated from, for example, aluminum oxide(Al₂O₃ or Al_(x)O where x may be not be an integer) or magnesium oxide.Ferromagnetic layers 122 and 124 may be fabricated from magneticmaterials, such as, for example, nickel-iron-cobalt (Ni—Fe—Co)compositions. The shield layers may be fabricated from, for example, asoft magnetic material such as nickel-iron (Ni—Fe). Back bias magnet 130may be fabricated from a permanent magnet material such as, for example,a cobalt-platinum (Co—Pt) alloy.

The operation of read head 110 according to one aspect of the inventionis described in conjunction with FIGS. 7A-7C. FIGS. 7A, 7B and 7C showtop views of read head 110 with magnetization vector 130′ of back biaslayer 130 oriented with respect to magnetization vectors 122′ and 124′of freelayers 122 and 124 to achieve optimum response of freelayers 122and 124 to external magnetic fields. In the absence of back biasmagnetization 130′, freelayer magnetization vectors 122′ and 124′ wouldbe antiparallel and parallel to ABS 160. Under the back bias ofmagnetization 130′, they arrange in a scissor orientation for optimumsensitivity. A benefit of the trapezoidal shape of back bias magnet 130is that the smaller base at trilayer reader stack 120 results inmagnetic flux concentration in that region resulting in deeperpenetration of the biasing field into reader stack 120 in the directionof the ABS.

FIGS. 7A-7C illustrate the effect of varying bit magnetizations onrecorded media on the magnetization directions 122′ and 124′ of firstfreelayer 122 and second freelayer 124 respectively. FIG. 7A showstrilayer reader stack 120 in a quiescent magnetic state when it is notunder the influence of magnetic flux emanating from recording media. Theangle of magnetization between first ferromagnetic freelayer 122 andsecond ferromagnetic freelayer 124 at the ABS is in a scissors relationfor optimum sensor response. FIG. 7B is a front view of read head 110showing trilayer reader stack 120 under the influence of a first stateof data D1 corresponding to a positive bit. This first state of datacauses the angle of magnetization between first freelayer 122′ andsecond freelayer 124′ to increase at the ABS. When this occurs, theresistance across trilayer reader stack 120 changes and is detected whena sense current is passed through trilayer reader stack 120. FIG. 7C isa top view of read head 110 showing trilayer reader stack 120 under theinfluence of a second state of data D2 corresponding to a negative bit.This second state of data causes the angle of magnetization betweenfirst freelayer 122′ and second freelayer 124′ to decrease at the ABS.As with the first state of data, the second state of data causes achange in resistance across trilayer reader stack 120 and is detectedwhen a sense current is passed through trilayer reader stack 120.

The operation of read head 110 is similar to that discussed for readhead 10 and schematically illustrated in FIG. 4A-4C, with one exception.The trapezoidal shape of trilayer reader stack 120 encourages a “C” typeof micromagnetic magnetization in freelayers 124 and 126. This forcesthe magnetization vectors into orientations parallel to the ABS anddiscourages the formation of “S” type micromagnetic magnetizationpatterns in the freelayers, thereby minimizing noise resulting from “C”type to “S” type switching behavior during operation.

The formation of reader 10 with trapezoidal back bias magnet 30 shown inFIGS. 2 and 3 is schematically illustrated in FIGS. 8A-8K. FIG. 8A showsa substrate coated with reader stack 220. The reader stack can be a GMRor a TGMR stack. In the next step, photoresist (PR) layer 260, coveringthe center portion of reader stack 220, is deposited as shown in FIG.8B. In the next step, shown in FIG. 8C, exposed reader stack 220 hasbeen removed by ion beam machining or etching or by other means known inthe art. Following removal of exposed reader stack 220, insulating layer250 is deposited on each side of reader stack 220 and PR layer 260 asshown in FIG. 8D. Insulating layer 250, as mentioned earlier, ispreferably aluminum oxide and is preferably deposited by atomic layerdeposition (ALD). In the next step permanent bias magnet 230 is thendeposited as shown in FIG. 8E comprising reader stack 220 with biasmagnets 230 above and below reader stack 220 separated from reader stack220 by insulating layers 250. The structure in FIG. 8E is then coveredwith PR layer 260 b with a narrow center width and wider ends as shownin FIG. 8F. The exposed structure not covered with PR layer 260 b isthen removed by ion beam machining or etching or other means known inthe art as shown in FIG. 8G. Insulator layer 250 is then deposited oneach side of the structure covered with PR layer 260 b as shown in FIG.8H. Side shields 240 and 242 are deposited to form the structure shownin FIG. 8I. Side shields 240 and 242 could be replaced with insulatorlayer 250 if needed. Removing PR layer 260 b in FIG. 8I reveals thestructure shown in FIG. 8J comprising rectangular reader stack 220separated from side shields 240 and 242 and trapezoidal bias magnets 230by insulating layer 250. Masking the top half of the structure shown inFIG. 8J and removing the remainder creates reader structure 10 shown inFIG. 8K comprising rectangular reader stack 220, side shields 240 and242 and trapezoidal back bias magnet 230 separated from each other byinsulating layer 250. Air bearing surface ABS is indicated in FIG. 8K.

The formation of reader 110 with trapezoidal back bias magnet 130 andtrapezoidal reader stack 120 shown in FIGS. 5 and 6 is schematicallyillustrated in FIGS. 9A-9K. FIG. 9A shows a substrate coated with readerstack 320. The reader stack can be a GMR or a TGMR stack. Photoresist(PR) layer 360, covering the center portion of reader stack 320, isdeposited as shown in FIG. 9B. In the next step, shown in FIG. 9C,exposed reader stack 320 has been removed by ion beam machining oretching or by other means known in the art. Following removal of exposedreader stack 320, insulating layer 350 is deposited on each side ofreader stack 320 and PR layer 360 as shown in FIG. 9D. Insulating layer350, as mentioned earlier, is preferably aluminum oxide and ispreferably deposited by atomic layer deposition (ALD). In the next step,permanent bias magnet 330 is then deposited as shown in FIG. 9Ecomprising reader stack 320 with bias magnets 330 above and below readerstack 320 separated from reader stack 320 by insulating layer 350. Thestructure in FIG. 9E is then covered with PR layer 360 b with a narrowcenter width and asymmetrically wider ends as shown in FIG. 9H. Theexposed structure not covered with PR layer 360 b is then removed by ionbeam machining or etching or other means known to produce the structureshown in FIG. 9G. Insulator layer 350 is then deposited on each side ofthe structure in FIG. 9G to produce the structure shown in FIG. 9H. Sideshields 340 and 342 are deposited on each side to form the structureshown in FIG. 9I. Side shields 340 and 342 could be replaced withinsulator layer 350 if needed. Removing PR layer 360 b in FIG. 9Kreveals the structure shown in FIG. 9J comprising trapezoidal readerstack 320, side shields 340 and 342 and trapezoidal bias magnet 330. Allare separated by insulating layer 350. Masking the top half of thestructure shown in FIG. 9J and removing the remainder creates readerstructure 110 shown in FIG. 9K comprising trapezoidal trilayer readerstack 320, side shields 340 and 342, and trapezoidal back bias magnet330 separated from each other by insulating layer 350. Air bearingsurface ABS is indicated in FIG. 9K.

While the invention has been described with reference to an exemplaryembodiment(s), it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment(s) disclosed, but that theinvention will include all embodiments falling within the scope of theappended claims.

1. A magnetoresistive sensor comprising: a trilayer sensor stackcomprising two ferromagnetic freelayers separated by a nonmagneticspacer; and a back biasing magnet adjacent a back end of the trilayersensor stack; wherein at least one of the trilayer sensor stack and theback biasing magnet has a shape that stabilizes a micromagnetic “C”state or concentrates magnetic flux in the trilayer sensor stack.
 2. Themagnetoresistive sensor of claim 1 wherein the back bias magnet has asubstantially trapezoidal shape.
 3. The magnetoresistive sensor of claim2 wherein the trilayer sensor stacks have a substantially trapezoidalshape.
 4. The magnetoresistive sensor of claim 1 wherein the trilayersensor stack has a substantially rectangular shape.
 5. Themagnetoresistive sensor of claim 1, wherein the nonmagnetic spacer layerof the trilayer sensor stack is an insulator layer and the trilayersensor stack is a tunneling magnetoresistive sensor.
 6. Themagnetoresistive sensor of claim 1, wherein the biasing magnet providesvertical bias to the trilayer sensor stack.
 7. The magnetoresistivesensor of claim 1, wherein the biasing magnet is a hard magneticmaterial.
 8. The magnetoresistive sensor of claim 7, wherein the hardmagnetic material is a cobalt-platinum based alloy or iron-platinumbased alloy.
 9. The magnetoresistive sensor of claim 1, wherein the backbiasing magnet is isolated from the trilayer sensor stack by aninsulating layer.
 10. The magnetoresistive sensor of claim 1, whereinthe ferromagnetic layers in the trilayer sensor stack are selected fromthe group consisting of nickel-iron, copper-iron, and nickel-iron-copperalloys.
 11. The magnetoresistive sensor of claim 1, and furthercomprising: lateral side shields adjacent both sides of the trilayersensor stack and the back biasing magnet.
 12. The magnetoresistivesensor of claim 11, wherein the lateral side shields are isolated fromthe trilayer sensor stack and the vertical biasing magnet by a sideshield insulating layer comprising aluminum oxide.
 13. Amagnetoresistive sensor comprising: a trilayer sensor stack comprisingtwo ferromagnetic layers separated by a nonmagnetic spacer layer, andhaving a front width proximate an air bearing surface and a back widthdistal from the air bearing surface; and a back biasing magnet adjacentthe back width of the trilayer sensor stack, the back biasing magnethaving a front width that is about the same as the back width of thetrilayer stack, and a back width; wherein at least the back biasingmagnet has a trapezoidal shape.
 14. The magnetoresistive sensor of claim13 wherein the back width of the trilayer sensor stack is larger thanthe front width of the trilayer stack.
 15. The magnetoresistive sensorof claim 13 wherein the back biasing magnet provides bias to thetrilayer sensor stack in a direction generally perpendicular to the airbearing surface.
 16. The magnetoresistive sensor of claim 13 wherein theback width of the biasing magnet is larger than its front width.
 17. Themagnetoresistive sensor of claim 13 and further comprising: lateral sideshields adjacent both sides of the trilayer sensor stack and the backbiasing magnet.
 18. The magnetoresistive sensor of claim 13 wherein theback width of the trilayer sensor stack is about the same as the frontwidth of the trilayer sensor stack.
 19. A magnetoresistive sensorcomprising: a trilayer sensor stack comprising first and second freelayers separated by a nonmagnetic spacer; a permanent magnet located onan opposite side of the trilayer sensor stack as an air bearing surface,the permanent magnet having a front side width less than a back sidewidth wherein a front side is closest to the trilayer sensor stack; afirst and second side shield adjacent the trilayer sensor stack andpermanent magnet; a top shield adjacent the first free layer; and abottom shield adjacent the second free layer.
 20. The magnetoresistivesensor of claim 19, wherein the trilayer sensor stack has a front sidewidth less than the back side width and a front side is closest to theair bearing surface.